The "power" of an air compressor

The subtext of this title is "how to cut through bulls**t claims about compressor performance in catalogues". There's a lot of deliberately misleading claims put out.

The "power" of a compressor is how much air it can pump at a given pressure in a given time. That is its output. At the bottom line, what a compressor does is supply atmospheric air but at a raised pressure. You cram more air into a given volume than is the there is at atmospheric pressure and supply this to whatever is demanding it.

The "Free Air Delivery" (FAD)

The above is known as the "Free Air Delivery" (FAD). There would be a problem quoting this if you did not standardise the pressure. Then even if you did, you would not be able to compare the fall-off in pumping capacity of a particular compressor with increasing output pressure. So this is what FAD is about:

You quote the FAD at a particular output pressure. That would normally be 90 p.s.i, which most air tools work at.

Imagine this - with the compressor running and supplying air at the stated pressure:

The volume of the air which is now back at atmospheric pressure is the Free Air Delivery. Divide that volume by the time it took for the compressor to pump it and you have the rate of FAD.

The traditional unit of the rate of FAD in Britain and America is Cubic Feet of air per Minute (c.f.m.).

You can expect certain FAD rates from certain classes of compressor. In the UK the most powerful single-phase motor you can connect to the domestic mains is 3HP (3 horse-power). A portable 3HP compressor will pump about 7 c.f.m. at 90 p.s.i.. An stationary good quality garage compressor will deliver about 10.5 c.f.m. at 90 p.s.i.. With a two-stage compressor with intercooling between the stages you can get a 25% higher FAD for the same motor power - an option which will cost you about 40% more but is for real.

How you "cheat" to give higher output figures

In at number one is quoting the displacement of the compressor, not the air it delivers. If you multiply the volume of the cylinder(s) by the number of times they reciprocate in one minute (= the crankshaft rotational speed), you will get the displacement. This is higher than the actual air delivered. This is because some of the air is not pushed into the tank of the compressor because of the back-pressure of air in the tank. Taking my compressor: its FAD is 7 c.f.m. at 90 p.s.i., whereas its swept volume is 11 c.f.m.. That is the typical ratio of FAD to swept volume for a single-stage compressor.

In at number two is quoting the FAD but at a lower pressure like 30 p.s.i.. That figure will be low double figures of percent higher than the FAD at 90 p.s.i., for a good compressor. If the compressor is not very good and struggles at a proper working pressure like 90 p.s.i. then the difference could be greater; a reason for quoting at this lower pressure.

Outright creating a fiction is another possibility

Even with true FAD, not all things are equal

Knowing that FAD at 90 p.s.i. is the standard for rating a compressor, you can create a very cheap machine which will sound just fine compared to the performance of a machine from a reputable manufacturer. But it won't last long. And here is how you do it. You take a cheaper smaller inadequately powerful motor and throw more load on it than it is supposed to take. Then you couple it to a smaller cheaper compressor (pump) and spin it at the maximum it will take in order to get a lot of air out of it. The machine is not going to last very long. You are paying a bit less for a lot less than if you got a proper well-specified machine.

(Richard Smith, June 2005)